Semiconductor devices are used in a variety of electronic applications, such as computers, cellular phones, personal computing devices, and many other applications. Home, industrial, and automotive devices that in the past comprised only mechanical components now have electronic parts that require semiconductor devices, for example.
Semiconductor devices are manufactured by depositing many different types of material layers over a semiconductor workpiece or wafer, and patterning the various material layers using lithography. The material layers typically comprise thin films of conductive, semiconductive, and insulating materials that are patterned and etched to form integrated circuits (IC's). There may be a plurality of transistors, memory devices, switches, conductive lines, diodes, capacitors, logic circuits, and other electronic components formed on a single die or chip.
With the semiconductor industry targeting smaller feature sizes, the interface regions between two adjacent thin films, and the surface properties of thin films, have become more important to device performance. The definition of an interface region between two films in terms of the thickness of the interface region, i.e., the number of atomic layers or molecular layers the interface region comprises, has become more critical as semiconductor devices are scaled down in size. In addition, there are limitations to the maximum temperatures allowed in thin film deposition processes because of device performance degradation.
In order to improve the interface region between two adjacent thin films, seed layers are often used. FIG. 1 shows a cross-sectional view of a prior art semiconductor device 100 comprising a workpiece 102. The workpiece 102 may comprise a semiconductor wafer, and may include a variety of material layers formed thereon, for example, metal layers, semiconducting layers, dielectric layers, diffusion barrier layers, etc., not shown. A first material layer comprising a metal layer 104 is formed over the workpiece 102, as shown. The metal layer 104 may comprise a gate of a transistor, a plate of a capacitor, a conductive line, or other electrical components or portions of electrical components of an integrated circuit, for example.
In many semiconductor designs, it is desirable to form a second material layer comprising an insulating or semiconductor material layer 114 over the metal layer 104. During the formation of the second material layer 114, an interface region 112 can form between the metal layer 104 and the insulating or semiconductor material layer 114, often comprising material of both the metal layer and the insulating or semiconductor material layer 114, for example. In some applications, this is undesirable, because the interface region 112 has a detrimental impact on the performance of the semiconductor device 100. It is the goal in many semiconductor designs to form an insulating or semiconductor material layer 114 directly abutting the metal layer 104, so that the bulk properties of the insulating or semiconductor material layer 114 and the metal layer 104 are achieved. Thus, often a seed layer 110 is formed on the metal layer 114 before depositing the material layer 114, as shown, to decrease the interface region 112 thickness.
One method of forming the seed layer 110 is by forming a monolayer of the atoms 108 of a desired species by chemisorption. Chemisorption is a process whereby an atom or molecule adheres to a surface through the formation of a chemical bond, rather than by physisorption. In physisorption, an atom or molecule adheres to a surface by a van der Waals type force or electrostatic attraction rather than by a chemical bond. Generally, chemisorption produces stronger bonds than physisorption.
A problem with forming a seed layer 110 comprising a monolayer of atoms 108 is that there is a limitation on the number of atoms 108 that may be formed on the top surface of the metal layer 104. This is because many of the atomic species that are important in semiconductor manufacturing absorb from the gas phase onto a metal surface by dissociative adsorption. In this process of dissociative adsorption, the first step is the adsorption of the molecule (like O2 or N2) on the surface of metal layer 104, and the second step is the dissociation of the molecule with each of the atoms 108 now being bound individually to the surface of metal layer 104. After a sufficient number of atoms 108 is adsorbed on the surface of metal layer 104 it becomes impossible for additional molecules from the gas phase to get close enough to the metal surface 104 to start the dissociative adsorption process, and the molecules bounce back from the surface. After access to the metal layer 104 surface is blocked in this way, the saturation coverage for the atomic/molecular species (e.g., of atoms 108) is reached. Typically the saturation coverage is well below one monolayer, i.e., where there would be a 1:1 relationship between atoms 106 and 108. For example, an ideal monolayer would have for each metal atom 106 in the metal layer 104 surface, one oxygen or nitrogen atom 108 adsorbed on the metal layer 104 surface.
However, a 1:1 monolayer is not actually formed; typical saturation coverages are well below 0.4 and often not more than 0.25 monolayer, which occurs because of a limited number of adsorption sites. The metal layer 104 has a number of atoms 106 disposed at the top surface. The atoms 106 at the top surface of the metal layer 104 have a fixed number of adsorption sites that may be occupied if adsorption of atoms 108 of the seed layer 110 proceeds via dissociative adsorption of molecules out of a gas phase, which is typically the process used to form the seed layer 110. For example, if the metal layer 104 comprises ruthenium (Ru) with crystal orientation 001, i.e., Ru (001), and atomic oxygen is the species to be formed as a seed on the metal layer 104, a seed layer 110 of a monolayer of oxygen atoms 108 having a density of 0.25 or less is achieved when the seed layer 110 is formed at room temperature by adsorption from a gas phase of molecular oxygen. In particular, in this example, there may be one oxygen atom 108 in the monolayer seed layer 110 for every four atoms 106 of the metal element of the material layer 104, as shown, resulting in a 1:4 ratio of the seed layer 110 atoms to the metal layer 104 atoms, or a seed layer 110 having density of 0.25 or less with respect to the density of the metal layer 104.
When the next material layer 114 is deposited, an interface region 112 is formed, comprising a thickness d1 of about 10 to 15 atomic layers. It is desirable for the interface region 112 to be as thin as possible, or more preferably, for no interface region 112 to form, in some applications.
What is needed in the art is a method of forming a material layer 114 over a metal layer 104 that results in the formation of a thinner interface region 112 between the material layer 114 and the metal layer 104. A well-defined interface between a metal layer 104 and a subsequently deposited material layer 114 is needed.
What is also needed in the art are improved methods of forming seed layers, in order to overcome the current limitations of interface and thin film engineering, and the limitations of kinetics of interface formation and thin film growth.